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Maintaining conveyor power processes

Power stations are 24/7 operations.

So ensuring they continue to run, no matter what, is a difficult enough proposition, but when these power stations are faced with the prospect of having to increase their output then making sure they are kept supplied creates a whole new raft of challenges.

As part of a push in the industry to become more sustainable whilst still delivering the same levels or higher of power, AGL's Loy Yang power station underwent a conveyor system upgrade to extend the machinery and minimise production stoppages, breakdowns, and unscheduled downtimes.

The Loy Yang mine itself is a massive operation that uses mainly automated processes to transport coal from the bottom of the pit to the nearby power station.

It employs massive bucket wheel excavators that exhume up to 4000 tonnes of coal per hour, after which it deposits the material on to dredgers conveyors that feed on to main transfer conveyors that run on each level of the mine.

These main transfer conveyor systems comprise multiple separate conveyors, each with a belt-width of two metres and a travel speed of 5.2 meters per second (approximately 19 kilometres per hour), and have a combined length of 25 kilometres.

The conveyors then move the coal to a raw coal bunker, which has an 80,000 tonne capacity.

Additional conveyors then move the coal from this bunker to two separate power stations located at the top of the mine, AGL's Loy Yang 2210MW and GDF Suez's Loy Yang B 1000MW power stations.

However, as the coal bunker only has enough capacity to fuel 20 hours of power generation, it is crucial that the conveying system consistently performs.

Working with Rockwell Automation, Loy Yang installed an upgrade in its four-level open cut coal mine, which included the redesign and progressive changeover of the ex-isting coal transfer conveyor system.

Keeping current

The original legacy transfer conveyor drive systems at the Loy Yang mine were based on water-cooled eddy-current coupling (ECC) technology.

While the ECCs were an ideal drive solution when first installed, as they provided high torque over a wide speed range – which is ideal for moving large amounts of coal from the bottom of a pit to the surface, it had become clear that these drives systems were now struggling to move the coal as effectively as possible.

On top of this, the existing eddy-current coupling drive systems were becoming more difficult to maintain while the control systems became unreliable.

"The ECCs are an older method of conveyor control and as the mine continued to grow we needed additional power so it was clear that we needed to move towards a more modern system," AGL Loy Yang mine electrical superintendent, Robert Collins, said.

"The need to keep up with increased power demand provided AGL Loy Yang with the opportunity to implement a drive solution incorporating the latest technology that would work effectively in the mine's rugged environment," he said.

To overcome this issue affecting the large numbers of conveyors on site, AGL Loy Yang focused on implementing new drive architecture technology in a gradual manner to allow for integration with existing drive systems and control architecture without heavily impacting production.

Working with Rockwell Automation, AGL Loy Yang developed a new drive solution built around an Allen-Bradley PowerFlex 7000 medium voltage AC drive.

Known as 'direct-to-drive' technology, it helps to eliminate the need for isolation transformer applications.

The mine's engineering team decided to equip each conveyor with a fully self-contained, cooled, and removable drive package that could be easily installed or uninstalled on all the site's conveyors.

The simplified nature of the drives eased the develop­ment of the portable drive packag-es.

"The transformerless configuration of the PowerFlex 7000 meant we were able to help minimise the footprint of the drive package," Rockwell engineering team leader, John Dunn, said.

"The main electrical feature is the load sharing between different drives on each conveyor; we developed a portioned stainless steel IP65-rated enclosure, equipped with an air-conditioned cooling system, to house each of the 6.6kV PowerFlex drives," he said.

"The 'minimal component count' was also a feature of the drive solution, with fewer parts eliminating the number of things that could go wrong."

Integration on site

Nine drive packages have been supplied to the coal mine to date.

Engineers overcame the integration issues caused by the existing drive technology and control architecture by interfacing the first drive packages with PLC5 systems packages in a neighbouring switchroom.

It then uses ControlLogix, which is linked to the mine's supervisory control and data acquisition (SCADA) system, via the mine's existing communications network.

Each drive package is then equipped with an Allen-Bradley electronic operator interface located on the front panel of the drive enclosure.

"The drives can be interrogated via these EOIs or through the mine's SCADA system," Dunn explained.

"The locally mounted EOIs make it easier for site personnel to access drive diagnostics without opening the dust-proof enclosures and exposing the drives to the elements."

The two teams also worked together to devise a strategy to seamlessly synchronise the new drive packages with the existing ECC drives.

"Integrating the new drives at the mine has been a relatively straightforward project to complete," he said.

"We have a lot of experience using the drives now, making it a fairly easy, run of the mill upgrade that doesn't take too long."

The end game

Since the installation of the drives Loy Yang has seen a reduction in production stoppages, breakdowns, and unscheduled downtimes.

Engineers now have advanced diagnostics and troubleshooting capabilities, allowing for a reduction in maintenance, and allows them to eliminate isolation transformers whilst providing load sharing between conveyors, which can now regenerate energy if required.

However the job is not yet done, with additional upgrades slated.

Following this project the AGL Loy Yang team is now planning to progressively replace other ECC drives across the mine, and is designing a new downhill conveyor based on the drive that will be used for backfilling the mine.

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